1. Field of the Invention
This invention relates to a microwave electron gun for use in a linear accelerator.
Linear accelerators that generate electron beams are employed not only in scientific research but also in medical and industrial fields, their applications including, for example, non-destructive testing and electron-beam lithography. Many of these linear accelerators use microwave energy: the electrons are emitted from an electron gun and accelerated to high velocities as they travel down an accelerating tube comprising a series of cavities containing microwave electric fields.
2. Description of the Background Art
The electron guns commonly employed in linear accelerators in the past comprise an anode which is held at ground potential, a cathode which is raised to a high negative potential, a heating filament disposed just behind the cathode, and a Wehnelt electrode surrounding the cathode. Heating of the cathode by the filament produces thermionic emission of electrons, which are accelerated toward the anode by the strong dc electric field that exists between the cathode and anode. The Wehnelt electrode focuses the electrons into a beam which passes through a hole in the anode and enters the accelerating cavity. The beam emittance depends on the intensity distribution of the electric field, and the properties of the beam can be controlled by altering the shape of the Wehnelt electrode.
One problem with the prior-art electron gun described above concerns the coupling between the electron gun and the accelerating tube. Microwave radiation tends to escape from the accelerating tube into the electron gun via this coupling, causing a loss of microwave energy, hence a reduction in the amount of energy available for accelerating the electrons.
Another problem is the complex structure of the Wehnelt electrode, which complicates the design of the electron gun. The complexity is a consequence of the high dc voltage that must be applied across the cathode and anode in order to direct the electrons into the accelerating tube.
A third problem is accurate positioning of the electron gun. This problem occurs because the electron gun is mounted separately from the accelerating tube.
An alternative type of electron gun employs microwave energy instead of a dc field to impart an initial acceleration to the electrons. The initial acceleration takes place in an electron-gun cavity disposed just in front of the cathode.
The development of such a microwave electron gun for use in a linear accelerator at Stanford University has been described in a paper by G. A. Westenkow and J. M. J. Madey in volume 2, part 2, pages 223 to 225 of Lasers and Particle Beams, published in 1984. With such a microwave electron gun, however, it is still necessary to solve the mounting problem described above, the problem of the loss of microwave energy, and other problems such as the shape of the electron-gun cavity and its linkage to the other cavities of the accelerating tube. The present invention is addressed to these problems.